Endothermic and exothermic reactions

In a nutshell

An exothermic reaction is a reaction where energy is released into the surroundings. An endothermic reaction is a reaction which absorbs energy from the surroundings. In this summary you will learn about both types of reactions with examples and reaction profiles.

Energy

Chemicals store energy. Different chemicals are able to store different amounts of energy.

The amount of energy in this universe will always be constant because energy cannot be created or destroyed. Energy can be transferred between chemicals and can be converted from one form to another.

Example

Kinetic energy can be converted into heat energy.

Exothermic reactions

Definition

Exothermic reactions release energy into the surroundings and arise when the products store less energy than the reactants.

Exothermic reactions usually release energy in the form of heat energy.

Exothermic reactions increase the temperature of the surroundings.

Examples

Combustion reactions, such as burning fuels, are very exothermic.
Neutralisation reactions, which involve acid and alkali, are exothermic.
Several oxidation reactions are exothermic
Self-heating cans of food have chemicals in the base: when these chemicals are released, exothermic reactions occur which release heat and heat the food.

Endothermic reactions

Definition

Endothermic reactions absorb energy from the surroundings and arise when the products store more energy than the reactants.

Endothermic reactions decrease the temperature of the surroundings.

Examples

Decomposition reactions are endothermic; heat needs to be supplied for endothermic reactions to take place.
Sporting injury packs involve endothermic reactions. When a pack is snapped, endothermic reactions take place inside which absorbs heat and make the pack feel cold. This can help with swelling.

Reaction profiles

Definition

Reaction profiles, sometimes referred to as energy level diagrams, show the energy of the reactants and the products. Reaction profiles can be used to determine if a reaction is endothermic or exothermic.

In reaction profiles, the difference between the energies of the reactants and products shows how much energy is released or absorbed in the reaction, this is the overall energy change.

Endothermic reaction profile	Exothermic reaction profile

A	Direction of reaction
B	Energy
1.	Activation energy
2	Energy absorbed
3.	Energy of reactants
4	Energy of products

A	Direction of reaction
B	Energy
1.	Activation energy
2.	Energy released
3,	Energy of reactants
4.	Energy of products

Activation energy

Definition

Activation energy is the minimum amount of energy required for reactants to react upon collision. If the reactants do not possess the activation energy, a reaction will not take place.

On a reaction profile, the activation energy is the difference between the energy of the reactants and the peak of the curve.

The higher the activation energy of a reaction, the more energy is needed for the reaction to take place.

Chemistry; Rates of reaction and energy changes; KS4 Year 10; Endothermic and exothermic reactions

1.	Energy of reactants
2.	Energy of products
3.	High activation energy
4.	Low activation energy